Solvent-Controlled Pd(II)-Catalyzed Aerobic Chemoselective Intermolecular 1,2-Aminooxygenation and 1,2-Oxyamination of Conjugated Dienes for the Synthesis of Functionalized 1,4-Benzoxazines

Article abstract of DOI:10.1021/acs.orglett.8b00352

Pd(II)-catalyzed intermolecular 1,2-aminooxygenation and 1,2-oxyamination of conjugated dienes have been developed. The chemoselective preparation of a variety of 2-functionalized and 3-functionalized 1,4-benzoxazine derivatives was accomplished via the a

Full text of DOI:10.1021/acs.orglett.8b00352

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Solvent-Controlled Pd(II)-Catalyzed Aerobic Chemoselective
Intermolecular 1,2-Aminooxygenation and 1,2-Oxyamination of
Conjugated Dienes for the Synthesis of Functionalized 1,4-
Benzoxazines
Ke Wen,† Zhengxing Wu,† Banruo Huang,† Zheng Ling,† Ilya D. Gridnev,‡ and Wanbin Zhang*,†
†Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs, School of Chemistry and Chemical Engineering, Shanghai Jiao
Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. China
‡Department of Chemistry, Graduate School of Science, Tohoku University, Aramaki 3-6, Aoba-ku, Sendai 980-8578, Japan
S
* Supporting Information
ABSTRACT: Pd(II)-catalyzed intermolecular 1,2-aminooxygenation and 1,2-oxyamination of conjugated dienes have been
developed. The chemoselective preparation of a variety of 2-functionalized and 3-functionalized 1,4-benzoxazine derivatives was
accomplished via the adjustment of a coordinating solvent. Oxygen was successfully used in this oxidative difunctionalization of
alkenes. Good yields and selectivities were obtained for most products. A product bearing a spiro structure was also obtained
from a 2,3-disubstituted-1,3-diene.
Scheme 1. Aminooxygenation and Oxyamination of
Conjugated Dienes Involving Palladium
ifunctionalization of alkenes is one of the most basic and
1−4
Defficient transformations in organic synthesis.
Among
these transformations, vicinal aminooxygenation and oxy-
amination of alkenes are very important processes2−4 because
of their broad use for the synthesis of catalysts and ligands,
biologically active molecules, and natural products.5 Over the
past few decades, a number of metal-catalyzed and metal-free
methods have been designed.2−4 Palladium, one of the most
commonly used metals, has also been applied to the vicinal
aminooxygenation and oxyamination of alkenes.4 In particular,
several palladium-catalyzed transformations have been devel-
oped for isolated alkenes, mainly using a Pd(II)/Pd(IV)
catalytic cycle in the presence of a strong oxidant such as
iodobenzene diacetate or hydrogen peroxide.4e−l However, in
studies concerning the difunctionalization of conjugated
dienes,6 aminooxygenation or oxyamination strategies are
limited because of the mechanistic differences with reactions
involving isolated alkenes. Even so, the issue is worthy of
research since the double bonds preserved in the difunction-
alization products can be further functionalized. Backvall
̈
previously reported a Pd(II)-mediated 1,4-oxyamination via a
Pd(II)/Pd(0) process. The reaction proceeds via the initial
formation of a π-allylpalladium intermediate; a nitrogen source
is then added to achieve the 1,4-oxyamination (Scheme 1a).4d
Despite these successful examples, there are currently no
reports concerning Pd(II)-catalyzed oxidative vicinal amino-
oxygenation or oxyamination of conjugated alkenes.7 In
addition, for the majority of palladium-catalyzed reactions,
selective control remains challenging (Scheme 1b).8 Further-
more, it is uncommon to use molecular oxygen as a terminal
Received: January 31, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00352
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
oxidant in Pd(II)-catalyzed oxidative difunctionalization re-
actions.9 During our research on the palladium-catalyzed
functionalization of alkenes,10 we developed a Pd(II)-catalyzed
chemoselective intermolecular 1,2-aminooxygenation and 1,2-
oxyamination of conjugated dienes, mainly via the adjustment
of a coordinating solvent (Scheme 1c). Molecular oxygen was
successfully used in the reaction as a terminal oxidant. A variety
of interesting functionalized 1,4-benzoxazine derivatives, a
structural motif found in numerous biologically active
molecules, were successfully prepared selectively.5c−f
Our study began with the reaction of N-tosyl protected 2-
aminophenol 1a with isoprene 2a, using 1,4-benzoquinone
(BQ) as the oxidant and 10 mol % Pd(OAc)2 as the catalyst.
The reaction was carried out in a series of solvents under a
nitrogen atmosphere at 70 °C for 24 h (Table 1, entries 1−6).
13−21) to achieve the chemoselective 1,2-aminooxygenation
and 1,2-oxyamination, respectively. When the oxidant was
changed to O2 (1 atm) in MeCN, the reaction proceeded with
a lower yield and a higher selectivity (entry 7). Catalytic
amounts of Cu(OAc)2 (20 mol %) had a negative effect on the
reaction (entry 8). Considering the solubility of isoprene (2a)
in the reaction system, we carried out the reactions in a sealed
tube. Fortunately, the reaction provided the desired product in
92% yield with 9/1 selectivity (entry 9). This may be due to the
increase of 2a in the reaction system in a sealed tube. A slightly
higher yield of 94% was obtained in a pressure vessel with 10
atm O2 (entry 10). Reducing the amount of Pd(OAc)2 led to a
corresponding reduction in yield (entry 11). Performing the
reaction in a sealed tube with air led to an obvious decrease in
the yield, indicating the importance of the concentration of
oxygen in the reaction mixture (entry 12). We also carried out
the reaction with the noncoordinating solvents DCE and
toluene in a sealed tube of O2 or a pressure vessel with O2 (10
atm) (see the Supporting Information). However, only a trace
amount of product was obtained in all cases, indicating the
importance of the coordinating solvents (MeCN and DMSO)
for this reaction. Using the promising reaction conditions
identified with MeCN, we first attempted the reaction in
DMSO using O2 at 1 atm (entry 13) or a high pressure (10
atm) (entry 14), but both results were unsatisfactory. We
therefore continued to optimize the reaction conditions using
BQ as the oxidant. In consideration of the Diels−Alder reaction
between 2a and BQ resulting in the consumption of isoprene, a
series of BQ reagents with different substituents were used for
the reaction (entries 15−18). With the exception of p-chloranil
(entry 16), other types of benzoquinones all led to improved
yields (entries 15, 17, and 18) compared with BQ (entry 6).
We next utilized 2,5-dimethyl-BQ and oxygen as oxidants to
further improve the yield (entries 19−21). The yield of product
4a increased to 78% using 2,5-dimethyl-BQ and oxygen (sealed
tube) as oxidants (entry 21).
With the above optimized reaction conditions (method A:
Table 1, entry 9; method B: Table 1, entry 21) in hand, a
variety of N-toluenesulfonyl-protected 2-aminophenols 1 were
studied to explore the substrate scope and selectivity (Table 2).
We first surveyed the reaction in MeCN using method A. For
5-substituted 2-aminophenol substrates with either electron-
donating or electron-withdrawing substituent groups, the
reaction proceeded well, giving the corresponding products in
good yields and selectivities (3b−h). Similar reactivity was
observed for 4-substituted 2-aminophenol substrates (3i−o). A
substrate possessing a 4,5-dimethyl group provided its
corresponding product in good yield and selectivity (3p). A
4,5-dichloro-substituted substrate (3q) provided poor reac-
tivity. For a 3-substituted 2-aminophenol substrate, low
reactivity and inverse selectivity were observed (3r), which
might be attributed to the effect of the steric bulk. For a
naphthalene substrate, the reaction provided a high yield and
good selectivity when method A was used (3s).
a
Table 1. Optimization of the Reaction Conditions
b
c
entry
solvent
DCE
oxidant
yield (%)
3a/4a
1
2
3
4
5
6
7
BQ
BQ
BQ
BQ
BQ
BQ
trace
trace
nd
trace
33
>20/1
>20/1
−
toluene
DME
1,4-dioxane
MeCN
DMSO
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
>20/1
5/1
27
<1/20
9/1
O2 (1 atm)
23
d
8
Cu(II) + O2 (1 atm)
O2 (sealed tube)
O2 (10 atm)
nd
92
94
−
9/1
9
10
8/1
e
11
O2 (sealed tube)
air (sealed tube)
O2 (sealed tube)
O2 (10 atm)
57
9/1
12
13
14
15
16
22
9/1
7
<1/20
<1/20
<1/20
<1/20
<1/20
<1/20
<1/20
<1/20
<1/20
7
duroquinone
38
p-chloranil
trace
54
f
17
g
BQ1
18
BQ2
53
19
20
21
BQ1 + O2 (1 atm)
BQ1 + O2 (10 atm)
BQ1 + O2 (sealed tube)
66
79
78
a
The reaction was carried out with 1a (0.1 mmol, 1 equiv), 2a (1
mmol, 10 equiv), and oxidant (0.2 mmol, 2 equiv) in the solvent (1
mL) in the presence of Pd(OAc)2 (10 mol %) at 70 °C for 24 h, unless
b
otherwise noted. Isolated yields of the two chemoselective products.
c
d
e
Determined by 1H NMR analysis. 20 mol % Cu(OAc)2 was used. 5
f
g
mol % Pd(OAc)2 was used. BQ1 = 2,5-dimethyl-BQ. BQ2 = 2,6-
dimethyl-BQ.
We observed that palladium black precipitated quickly in
noncoordinating solvents (DCE and toluene) and polar
solvents (DME and 1,4-dioxane) (entries 1−4). To our delight,
in the coordinating solvent MeCN, the oxidative cyclization
proceeded smoothly to give the desired 2-functionalized
product 3a with moderate selectivity (entry 5). Intriguingly,
the 3-functionalized product 4a was obtained with high
selectivity with the coordinating solvent DMSO (entry 6).
These results indicate that the coordination between palladium
and the coordinating solvent MeCN or DMSO could facilitate
the reactions. We therefore optimized the reaction conditions
using the solvents MeCN (entries 7−12) and DMSO (entries
The reaction was also investigated in DMSO using method
B, as shown in Table 3. For 5-substituted 2-aminophenol
substrates with either electron-donating or electron-with-
drawing substituent groups, chemoselective control could be
achieved using method B. The corresponding products were
obtained in good yields and selectivities (4b−h). For 4-
substituted 2-aminophenol substrates, chemoselective control
was also generally achieved in good yields and selectivities (4i−
o). A substrate bearing a 4,5-dimethyl group also gave a
B
DOI: 10.1021/acs.orglett.8b00352
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ab
,
a b
,
Table 2. Scope of 2-Aminophenols in MeCN
Table 4. Scope of Conjugated Dienes
a
The reactions were carried out with 1a (0.1 mmol, 1 equiv) and 2 (1
mmol, 10 equiv) in the solvent (1 mL) in the presence of Pd(OAc)2
(10 mol %) at 70 °C for 24 h. Method A: the reaction was carried out
in MeCN under oxygen in a sealed tube. Method B: the reaction was
carried out in DMSO with 2,5-dimethyl-BQ (2 equiv) under oxygen in
a
The reactions were carried out with 1 (0.1 mmol, 1 equiv) and 2a (1
mmol, 10 equiv) in the presence of Pd(OAc)2 (10 mol %) in MeCN
(1 mL) at 70 °C for 24 h under oxygen in a sealed tube. Isolated
yields of the two chemoselective products are shown. The ratios were
determined by H NMR analysis. The reaction was performed with
O2 (10 atm). 5 mol % Pd(OAc)2 was used.
b
b
a sealed tube. Isolated yields of the two chemoselective products are
c
1
c
shown. The ratios were determined by 1H NMR analysis. Conjugated
d
d
dienes 2c and 2d (3 equiv). Conjugated diene 2e (5 equiv).
e
Conjugated diene 2f (5 equiv) and O2 (20 atm).
a b
,
Table 3. Scope of 2-Aminophenols in DMSO
with high selectivity (entry 2, 4t). For 2,3-dimethylbuta-1,3-
diene (2c), the corresponding 2-functionalized product was
obtained in high yield and selectivity using method A (entry 3,
3u). Two isomers were obtained in 73% total yield with an
approximately 1:1 ratio using method B (entry 3, 4u).
Interestingly, for 2,3-disubstituted-buta-1,3-diene 2d, the spiro
product 3v (entry 4) was obtained in 68% yield using method
A. For (E)-penta-1,3-diene (2e), the diene underwent
conversion in 67% yield with 2:1 selectivity using method A
(entry 5, 3w). Interestingly, internal difunctionalization of the
diene occurred using method B, giving the corresponding
product in 58% yield (entry 5, 4w). For the ring compound 1,3-
cycloheptadiene (2f), the reaction proceeded well in 65% yield
with 2:1 selectivity using method A (entry 6, 3x).
The 2-functionalized and 3-functionalized products were able
to partake in several transformations using different reaction
conditions. Compound 5 was obtained through dihydroxylation
of alkene 3a in 58% yield, along with a small amount of ketone
product (Scheme 2a). Additionally, the catalytic product 4t
could be oxidized to its corresponding acid 7 in 68% yield,
which could then be reduced to afford a prostaglandin D2
receptor antagonist5f in 88% yield (Scheme 2b). In addition, 4t
a
The reaction was carried out with 1 (0.1 mmol, 1 equiv), 2a (1 mmol,
10 equiv), and 2,5-dimethyl-BQ (2 equiv) in the presence of
Pd(OAc)2 (10 mol %) in DMSO (1 mL) at 70 °C for 24 h under
b
oxygen in a sealed tube. Isolated yields of the two chemoselective
products are shown. The ratios were determined by 1H NMR analysis.
c
d
The reaction was performed with O2 (10 atm). 5 mol % Pd(OAc)2
was used.
Scheme 2. Further Derivation of 2-Functionalized and 3-
Functionalized 1,4-Benzoxazine Derivatives
promising result (4p). However, a 4,5-dichloro-substituted
substrate gave its corresponding product in only moderate yield
(4q). The effect of the steric bulk was also significant, as
evidenced by the use of a 3-substituted 2-aminophenol
substrate. The desired product was obtained in low yield but
with good selectivity using method B (4r). High yield and good
selectivity were observed for a naphthalene substrate (4s).
We next turned our attention to investigating the scope of
conjugated dienes (Table 4). We found that for linear dienes, in
addition to isoprene (2a) (entry 1), 1,3-butadiene (2b) also
underwent conversion in good yield with moderate selectivity
using method A (entry 2, 3t). The inversion of chemo-
selectivity was also achieved using method B in moderate yield
C
DOI: 10.1021/acs.orglett.8b00352
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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with difficulty.
In summary, we have developed a Pd(II)-catalyzed aerobic
intermolecular chemoselective 1,2-aminooxygenation and 1,2-
oxyamination of conjugated dienes for the preparation of a
variety of 2-functionalized and 3-functionalized 1,4-benzoxazine
derivatives in different systems. The oxidative difunctionaliza-
tion of conjugated dienes was achieved using oxygen as oxidant.
For most of the products, good yields and selectivities were
obtained.
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ASSOCIATED CONTENT
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* Supporting Information
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ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00352.
Nordberg, R. E.; Zetterberg, K.; Åkermark, B. Organometallics 1983, 2,
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supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by e-mailing data_request@ccdc.cam.ac.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wanbin@sjtu.edu.cn.
ORCID
(6) For selected palladium-catalyzed difunctionalizations of con-
jugated alkenes, see: (a) Backvall, J.-E.; Nordberg, R. E. J. Am. Chem.
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H.; Zhao, B.; Shi, Y. J. Am. Chem. Soc. 2007, 129, 762. (e) Piera, J.;
Wanbin Zhang: 0000-0002-4788-4195
Notes
Persson, A.; Caldentey, X.; Backvall, J.-E. J. Am. Chem. Soc. 2007, 129,
̈
The authors declare no competing financial interest.
́
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Lloyd-Jones, G. C.; Booker-Milburn, K. I. J. Am. Chem. Soc. 2008, 130,
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(7) Just before our submission, a relevant reference concerning 1,2-
aminooxygenation was published as a just-accepted paper. See: Shen,
H.-C.; Wu, Y.-F.; Zhang, Y.; Fan, L.-F.; Han, Z.-Y.; Gong, L.-Z. Angew.
Chem., Int. Ed. 2018, 57, 2372.
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ACKNOWLEDGMENTS
■
This work was partially supported by the National Natural
Science Foundation of China (21472123 and 21620102003),
the Science and Technology Commission of Shanghai
Municipality (15JC1402200), and the Shanghai Municipal
Education Commission (201701070002E00030).
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